Is it possible to get back to `Add mode`?
The options at the bottom of the Tool Shelf (left Panel in the 3D view) and the 'F6 menu' only exist when you're adding an object.  

If you add an object (Cube, UV Sphere, Torus...) and press enter to accept the addition, you're too late in changing the options with the F6 menu.  

Shift+A > Add Torus, press F6 or T, now you can change the settings, before you press enter or carry out a further action.  

Objects you add are not procedural, you cannot just change their parameters at any time,but only when you add them to your scene.